[yocto] How to make a recipe doing mkimage on a initrd image?

[Hans Beckerus]

I have created a cpio type rootfs image and are trying to load it as a 
initrd ramdisk.
After several boot failures  (U-Boot complains it is not a valid image) 
I finally realized that the U-Boot version being used on this system 
requires that the ramdisk image is also prepended with a mkimage header. 
The standard image .bbclass that I am using in my simple recipe does not 
seem to have this functionality. Has anyone done a recipe (or image 
class?) that is calling mkimage in the do_deploy stage that could help 
me on the way here?
I wish to have it work similar to the kernel uImage build for which I 
guess the u-boot-native-mkimage is being used.

Hans